1. Field of the Invention
The present invention relates to determining the location of communications devices in a network communications system. More particularly, the present invention utilizes global positioning system (GPS) satellites to track cellular telephone positions in a cellular telephone network.
2. Description of Related Art
In recent years, cellular telephones have increasingly been used as a safety device to call for help when assistance is needed. However, the effectiveness of such systems has been reduced by the difficulty in identifying a caller""s position. Presently, a caller recites in detail a location, such as an address, to guide emergency workers such as police, fire or medical personnel to the site where assistance is needed. In emergency situations, the caller may be unable to accurately identify a position. Furthermore, when a caller is moving, the position where assistance is needed keeps changing. Thus, a need exists for a system to automatically determine the location of a mobile communications device such as a cellular telephone placing a call.
Global positioning systems (GPS) are used to identify the location of objects. These systems frequently rely on complex circuitry to decipher codes transmitted by three or more GPS satellites which orbit the earth. Such complicated circuitry consumes battery power and adds to device cost, both undesirable attributes in a hand-held mobile telephone.
Determining an object location using GPS signals requires precise timing. GPS receivers measure the delay between a transmission of a GPS signal from a GPS satellite to a receiving of the GPS signal at a receiving unit to determine the distance from the receiving unit to the satellite. GPS satellites include accurate clocks and encode the time they begin transmitting each god unit of GPS data in each broadcast. The receiving unit records the time the signal is received. An elapsed time between signal transmission and signal reception divided by the propagation speed of the GPS signal determines the distance between the satellite and the receiving unit. By computing three distances or xe2x80x9cpseudorangesxe2x80x9d from three satellites such as satellites 104, 108, 112 of FIG. 1, the position of a receiving unit such as mobile communications device 134 can be determined.
One difficulty with GPS systems is that the satellite clock and a timing apparatus such as a receiving clock need to be carefully synchronized. GPS satellites carry both rubidium and cesium atomic clocks to maintain an accurate time frame. Although these clocks are very accurate, the atomic clocks are physically left to drift off a standard xe2x80x9cGPSxe2x80x9d time system. The amount of drift is typically kept within a millisecond. The drift is permitted because it is difficult to interfere with precise atomic clocks without introducing inaccuracies. However, the performance of the clock is carefully monitored by a centralized control center and the amount of drift is accurately known. The amount of drift is included in information broadcast by each GPS satellite. This error information is transmitted as a co-efficient of a second order polynomial given by:
DT=A0+A1(Txe2x88x92T0)+A2(Txe2x88x92T0)2xe2x80x83xe2x80x83(1)
Where T0 is a time epoch, A0 is a satellite clock time offset, A1 is a fractional frequency offset, and A2 is a fractional frequency drift.
A second source of errors results from random drift characteristics. Random drift characteristics are not predictable, such random drifts are observable only over longer periods of time. When the base station does not receive signals directly from the GPS satellites, the longer periods of time increase the sample size needed from the cellular telephone. However, when the base station directly receives data from the GPS satellites, the base station can correct for random drift characteristic.
A receiving time is also needed to compute a pseudorange. The receiving time is the time at which the GPS signal is received. Various methods may be used to record a receive time including the use of an internal high quality quartz crystal clock or using external timing from an atomic standard clock. Alternately, when four GPS satellite signals are available, a four polynomial equation may be solved for four unknown variables, three variables representing position in three dimensional space and a fourth variable representing the time element. These four variables may be expressed in equations 2, 3, 4, 5 as follows:
Pk1={square root over ((u1xe2x88x92uk+L )2+L +(v1xe2x88x92vk+L )2+L +(w1xe2x88x92wk+L )2+L )}+c dtkxe2x80x83xe2x80x83(2)
Pk2={square root over ((u2xe2x88x92uk+L )2+L +(v2xe2x88x92vk+L )2+L +(w2xe2x88x92wk+L )2+L )}+c dtkxe2x80x83xe2x80x83(3)
Pk3={square root over ((u3xe2x88x92uk+L )2+L +(v3xe2x88x92vk+L )2+L +(w3xe2x88x92wk+L )2+L )}+c dtkxe2x80x83xe2x80x83(4)
Pk4={square root over ((u4xe2x88x92uk+L )2+L +(v4xe2x88x92vk+L )2+L +(w4xe2x88x92wk+L )2+L )}+c dtkxe2x80x83xe2x80x83(5)
Pk is a psuedorange (Pk) which can be solved for the four unknown variables, uk, vk, wk, dtk where (uk, uk, wk) is the position of the kth GPS satellite (U, V, W) is the position of the receiving unit, dtk is time elapsed from transmission to reception, and c is the speed of light. A description of these equations is provided on pages 204 to 217 of a book entitled GPS Satellite Surveying by Alfred Leick published in 1990.
A second method of determining time of transmission and time of reception uses carrier phases. Alternately, Doppler phase shifts may be used. However, these techniques suffer from the difficulty in obtaining a correct cycle, commonly called a cycle ambiguity. Such techniques and the associated difficulties are described on page 209 to 217 of the book GPS Satellite Surveying. 
U.S. patent application Ser. No. 5,379,224, entitled GPS Tracking System by Brown, et al., describes a low cost tracking system in which a GPS sensor samples a received GPS signal and re-transmits the samples to a central work station. The central work station described in Brown performs the GPS processing. However, the system described in Brown uses significant bandwidth to transmit sufficient data to enable an accurate position fix on the GPS sensor. The bandwidth allocated to each cellular telephone is limited.
Thus, there exists a need for a system which allows the cellular telephones to be located when an emergency arises, The system is preferably inexpensive, does not utilize significant cellular bandwidth, and is robust to handle changes in the need for GPS information when an emergency situation arises.
The present invention describes a system to track the position of a mobile communications device. The mobile communications device includes circuitry which receives and samples GPS signals from a plurality of satellites. An internal timing apparatus synchronized with a base station clock records the time at which the GPS signal is received. The mobile communications device transmits a packetized sample of the GPS signal and a time stamp indicating when the sampled GPS data was received to the base station. By utilizing the time stamp and the GPS data, a processing circuit in the base station determines the position of the mobile communications device.